Individual Submission L. Dusseault
Internet-Draft OSAF
Expires: February 6, 2005 August 8, 2004
Partial Document Changes (PATCH Method) for HTTPdraft-dusseault-http-patch-04
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 6, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Several applications extending HTTP [3] require a feature to do
partial resource modification. Existing HTTP functionality only
allows a complete replacement of a document. This proposal adds a
new HTTP method, PATCH, to modify an existing HTTP resource.
Dusseault Expires February 6, 2005 [Page 1]

Internet-Draft HTTP PATCH August 20041. Introduction
Three use cases initially motivated this proposal
1. WebDAV [2] is used by authoring applications to store and share
files on the internet. For example, Adobe Photoshop has a
Workgroup feature allowing the user to browse a repository and
save the file. Currently, Photoshop only publishes the file to
the repository rarely, because Photoshop files are typically
large and upload is slow. Worse, large uploads are more likely
to be interrupted. Although HTTP provides byte range downloads,
it does not provide a mechanism for partial uploads.
2. DeltaV [6] extends WebDAV to do versioning. In versioning
environments, a large number of files may be updated with very
small changes. For example, a programmer may change the name of
a function used in a hundred source files. Versioning
applications typically send deltas or patches to the server to
modify these files, however DetaV does not yet have this
functionality.
3. The SIMPLE WG is devising a way to store and modify configuration
information. The biggest feature missing from HTTP is the
ability to modify information in a very lightweight manner, so
that the client that decides to change its presence state from
"free" to "busy" doesn't have to upload a large document. This
can be accomplished through changes to a HTTP resource as well.
Other working groups (like netconf) are also considering manipulating
large files using HTTP GET and PUT. Sometimes the files aren't that
large but the device is small or bandwidth is limited, as when phones
need to add a new contact to an address book file. This feature
would allow much more efficient changes to files.
This specification defines a new HTTP 1.1 method for patches. A new
method is necessary to improve interoperability and prevent errors.
The PUT method is already defined to overwrite a resource with a
complete new body, and MUST NOT be reused to do partial changes.
Otherwise, proxies and caches and even clients and servers may get
confused as to the result of the operation.
Note that byte ranges are already used in HTTP to do partial
downloads (GET method). However, they are not defined for uploads,
and there are some missing pieces for uploads. For example, the HTTP
specification does not define a particularly informative error to
send if the byte range in a PUT is invalid. Byte ranges (or some
other kind of range) could be made to work in this specification but
a more flexible mechanism (one that could also encompass XML patch
formats) was desired, as well as a method that would not confuse
caching proxies. Reliable and tested patch algorithms already exist,
Dusseault Expires February 6, 2005 [Page 2]

Internet-Draft HTTP PATCH August 2004
and this specification takes advantage of that existing work.
Other patch formats ("delta encodings") are defined for HTTP in RFC3229 [4]. That specification defines delta encodings for cache
updates, not for user write operations. It does mean that servers
can reuse delta encoding algorithms to support both that
specification and this proposal.
This specification defines the new method PATCH to alter a single
existing resource, in place, by applying a patch. The operation is
atomic. Note that WebDAV MOVE and COPY requests, if supported by the
HTTP server, can be useful to independently rename or copy a whole
resource before applying PATCH to either the source or destination
URL to modify the contents.
Dusseault Expires February 6, 2005 [Page 3]

Internet-Draft HTTP PATCH August 20042. Patch Formats
A set of changes for a resource is itself a document, called a patch
document. The patch format is uniquely identified through a MIME
type. Servers advertise supported patch formats by advertising these
MIME types, and clients specify which one they're using by including
the MIME type in the request. MIME types were specifically chosen so
that there would be a well-defined way for other PATCH extensions to
define their own patch formats and how to use them.
This specification only defines usage of the platform-portable gdiff
[9] format identified as 'application/gdiff'. Servers SHOULD support
gdiff for all authorable resources, that is all resources that
support PUT. Some requirements apply only to specific patch formats,
and in this specification those requirements are spelled out only for
gdiff.
Dusseault Expires February 6, 2005 [Page 4]

Internet-Draft HTTP PATCH August 20043. Mechanisms3.1 PATCH Method
The PATCH method requests that the request body (a patch document) be
applied to the resource identified by the Request-URI. The server
MUST NOT create a new resource with the contents of the request body,
although it MAY (depending on the patch document format) apply the
request body to an empty entity to result in the content for the new
resource. The target resource's content type MUST be one to which
the patch format applies. The server MUST apply the entire patch
atomically and never provide (e.g. in response to a GET during this
operation) a partially-patched body. If the entire patch file cannot
be successfully applied then the server MUST fail the entire request,
applying none of the changes. See error handling section for details
on status codes and possible error conditions.
In the model defined in RFC3230 [5], the patch document is modelled
as an instance being sent to the server. Thus, if the server
supports instance manipulations, the client MAY apply these
manipulations to the patch document after it is generated (for
example, a compression algorithm). On the receiving end, the server
MUST undo the instance manipulation then apply the resulting document
as a patch.
PATCH request bodies MUST NOT be cached. A cache MAY mark the
resource identified in the Request-URI as stale if it sees a
successful response to the PATCH request.
The PATCH request MUST have a body. It MUST include the Content-Type
header with a MIME [1] type value identifying the patch format used
in the request body. The request body MUST be in some format which
has the semantics of defining a change to an existing document.
The PATCH request is subject to access control, which in turn may
require authentication. If the server supports WebDAV Access Control
[8], then the PATCH request SHOULD be subject to the same access
control permissions as the PUT request.
If the gdiff format is used:
o The client MUST verify that it is applying the patch document to a
known entity. There are two reliable ways to do this. The first
way is to find out the resource ETag at the time the body is
downloaded, and use that Etag in the If-Match header on the PATCH
request to make sure the resource is still unchanged. The second
way to use WebDAV LOCK/UNLOCK to reserve the file (first LOCK,
then GET, then PATCH, then UNLOCK). Gdiff collisions from
Dusseault Expires February 6, 2005 [Page 5]

Internet-Draft HTTP PATCH August 2004
multiple users are more dangerous than PUT collisions, because a
gdiff that is not operating from a known base point may corrupt
the resource. Therefore, if neither strong ETags nor LOCKS are
available from the server, the client MUST use If-Unmodified-Since
as a less-reliable safeguard.
o If the Request-URI does not identify an existing resource, the
server SHOULD (subject of course to access control and other
restrictions) create a resource with an empty body and apply the
gdiff changes to that empty entity. A client SHOULD verify that
the URL is unmapped, as expected, with use of the "If-None-Match:
*" header.
Simple PATCH example
PATCH /file.txt HTTP/1.1
Host: www.example.com
Content-type: application/gdiff
If-Match: "e0023aa4e"
Content-Length: 100
[gdiff-binary-body]
Figure 1
This example illustrates use of the gdiff algorithm on an existing
text file.
3.2 PATCH Response3.2.1 Success Response
A successful response with the 204 No Content status code implies
that no new resource was created. A successful response with the 201
Created status code informs the client that a new resource was
created.
The server SHOULD provide a MD5 hash of the resource entity after the
patch was applied. This allows the client to verify the success of
the operation. As with PUT, the PATCH method MUST cause the ETag to
change if the resulting entity is not identical to the original. If
the server supports strong ETags, the server MUST return a strong
ETag for use in future client operations. The server SHOULD return
the Last-Modified header in any case, but the server MUST return the
Last-Modified header if ETags aren't supported.
Dusseault Expires February 6, 2005 [Page 6]

Internet-Draft HTTP PATCH August 2004
Successful PATCH response to existing text file
HTTP/1.1 204 No Content
Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
ETag: "e0023aa4e"
3.2.2 Error handling
This proposal uses the same mechanism as DeltaV (defined in section1.6 of RFC3253) to add much-needed info to base HTTP error responses.
Existing HTTP status codes are not infinitely extensible but XML
elements and namespaces are more so, and it's simple to treat the
HTTP error code as a rough category and put detailed error codes in
the body. Clients that do not use the extra information ignore the
bodies of error responses.
The PATCH method can return the following errors. Please note that
the notation "DAV:foobar" is merely short form for expressing "the
'foobar' element in the 'DAV:' namespace". It has meaning only in
this specification, not on the wire. Also note that the string error
codes are not meant to be displayed but instead as machine parsable
known error codes (thus there is no language code).
DAV:delta-format-unsupported: Used with 403 Forbidden status code.
Returned by the server when it doesn't support the patch format
chosen by the client.
DAV:delta-format-forbidden-on-resource: Used with 403 Forbidden when
the patch format chosen by the client is supported by the server
but not allowed on this kind of resource.
DAV:delta-format-badly-formatted: Used with 400 Bad Request when the
server finds that the patch document provided by the client was
badly formatted or non-compliant. The definition of badly
formatted or non-compliant depends on the patch format chosen, but
generally if the server finds it can't handle the current patch
even though it supports the format used, this error ought to be
appropriate.
DAV:patch-empty-resource: Used with 409 Conflict when the resource
addressed in the Request-URI exists but is empty, and the patch
format cannot be applied to an empty document. Note that some
patch formats may be applied to an empty document, in which case
this error wouldn't be used.
Dusseault Expires February 6, 2005 [Page 7]

Internet-Draft HTTP PATCH August 2004
DAV:patch-result-invalid: Used with 409 Conflict when the resource
could be patched but the result of the patch would be a resource
which is invalid. This could mean, for example, that a XML
resource would become an invalid XML file if the patch specified
that a close element text line should be deleted.
"404 Not Found" can be used (with no body/error element) when the URL
in by the Request-URI does not map to a resource and the server
cannot apply the patch document to a new empty resource (thus this
error wouldn't be used with gdiff patch documents).
3.3 Advertising Support in OPTIONS
The server advertises its support for the features described here
with OPTIONS response headers. The "Allow" OPTIONS header is already
defined in HTTP 1.1 to contain all the allowed methods on the
addressed resource, so the server MUST add PATCH if it is allowed.
Clients also need to know whether the server supports special patch
formats, so this document introduces a new OPTIONS response header
"Accept-Patch". "Accept-Patch" MUST appear in the OPTIONS response
for any resource where the PATCH method is shown as an allowed
method.
OPTIONS * is not used to advertise support for PATCH because the
patch formats supported are likely to change from one resource to
another. A server MAY include the Accept-Patch header in response to
OPTIONS *, and its value MAY be the union of known supported patch
formats.
Accept-Patch = "Accept-Patch" ":" #media-type
Example: OPTIONS request and response for specific resource
[request]
OPTIONS /example/buddies.xml HTTP/1.1
Host: www.example.com
[response]
HTTP/1.1 200 OK
Allow: GET, PUT, POST, OPTIONS, HEAD, TRACE, DELETE, PATCH
Accept-Patch: example/xcap+xml, application/gdiff
Dusseault Expires February 6, 2005 [Page 8]

Internet-Draft HTTP PATCH August 2004Appendix A. Acknowledgements
PATCH is not a new concept, it first appeared in HTTP in drafts of
version 1.1 written by Roy Fielding and Henrik Frystyk.
Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm, Scott
Lawrence, Jeffrey Mogul, Roy Fielding, Greg Stein, Jim Luther, Alex
Rousskov and Jamie Lokier for review and advice on this document.
Dusseault Expires February 6, 2005 [Page 11]

Internet-Draft HTTP PATCH August 2004Appendix B. ChangesB.1 Changes from -00
OPTIONS support: removed "Patch" header definition and used Allow and
new "Accept-Patch" headers instead.
Supported patch formats: removed vcdiff and diffe as these do not
have defined MIME types and did not seem to be strongly desired.
PATCH method definition: Clarified cache behavior.
B.2 Changes from -01
Removed references to XCAP - not yet a RFC.
Fixed use of MIME types (this "fix" now obsolete)
Explained how to use MOVE or COPY in conjunction with PATCH, to
create a new resource based on an existing resource in a different
location.
B.3 Changes from -02
Clarified that MOVE and COPY are really independent of PATCH.
Clarified when an ETag must change, and when Last-Modified must be
used.
Clarified what server should do if both Content-Type and IM headers
appear in PATCH request.
Filled in missing reference to DeltaV and ACL RFCs.
Stopped using 501 Unsupported for unsupported patch formats.
Clarified what a static resource is.
Refixed use of MIME types for patch formats.
Limited the scope of some restrictions to apply only to 'gdiff'
usage.
B.4 Changes from -03
Various typographical, terminology consistency, and other minor
clarifications or fixes.
Dusseault Expires February 6, 2005 [Page 12]

Internet-Draft HTTP PATCH August 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Dusseault Expires February 6, 2005 [Page 13]